L'ISLE-VERTE, Quebec, Jan 31 (Reuters) - Investigators sifting through the charred debris at a Quebec home for senior citizens may have found the spot where the fire started, police said on Friday, as the grief-stricken town prepared for a special ceremony for the victims on Saturday that will include Prime Minister Stephen Harper.

Thirty-two elderly residents are presumed dead after a fire ripped through the three-story building early on Jan. 23 in the tiny town of L'Isle-Verte on the south shore of the St. Lawrence River, about 230 km (143 miles) northeast of Quebec City.

It was the second calamity to hit a small Quebec town in the past seven months. Last July, a crude oil tanker train derailed and exploded in Lac-Megantic, killing 47 people.

Quebec police spokesman Michel Brunet said the search crews halted their work on Friday morning after finding what they believe could potentially be important evidence for the probe into the cause of the fire.

They will not proceed until a judge grants them a warrant to do so, he said, calling it a routine precaution to ensure any evidence is handled properly and will be admissible in court if necessary.

"We are working with extreme care, and we've discovered certain clues that lead us to believe that the zone we're currently working in is a critical zone which may, and I stress may, be where the fire started," Brunet said.

Police have requested that a chemist and an electrician come to the scene to analyze the findings, he said, but declined to provide further details.

Despite media reports that a cigarette or an electrical problem may have ignited the blaze, police have said it's too early to speculate.

"All theories are being considered, and we're simply trying to act in compliance with the law," Brunet said.

The turning point in the search came after 24 bodies had been recovered from the ruins. Eight remain missing.

Investigators have worked in the brutal cold and whipping winds to scour about three-quarters of the site, which sits in the middle of the town on the main street. They brought in special de-icing machines to melt the thick ice that has encased all the debris and bodies as a result of the huge amount of water sprayed from fire hoses.

A giant red-and-white striped tarp, resembling a circus tent, covers the area where they are working to keep it warm.

POLITICIANS TO ATTEND MASS

The usually tranquil town, a tourist attraction known for its nature sanctuary and salt marsh-fed lamb, will be invaded by politicians from Ottawa and Quebec City on Saturday for a commemorative mass in the town's simple Roman Catholic church.

On the eve of the event, to be televised nationally, parishioners bustled around the church and put the final touches on the decor.

"We expect about 1,000 people," one organizer said, watching with some trepidation as journalists set up television cameras and lights and unrolled cables.

In addition to Harper, those attending include opposition leader Thomas Mulcair, federal cabinet ministers, Quebec Premier Pauline Marois and Governor General David Johnston. Pope Francis sent his condolences to L'Isle-Verte on Thursday.

Some 20 framed photographs of the victims hung on a board at the front of the church, framed by little paper cut-out hearts. A letter from an employee at the senior citizens home was pinned to the board and read: "To my 32 angels ... I will never forget you."

A child's drawing of a large multi-colored heart was dedicated to "all the grandparents." Next to it were photographs of school children visiting the elderly in their home.

"When you live in a small village like L'Isle-Verte, the community spirit, the family spirit, is very strong. When someone suffers, everyone suffers with them," said Odette Bernatchez, a church worker helping to organize the ceremony.

"These were people's neighbors, their cousins, uncles, aunts, grandparents, so the whole population really feels it in their heart," she said.

The town residents have had nothing but praise for the owners of the senior citizens home, which health officials said was fully compliant with regulations and had not received any complaints.

But it was only partially equipped with sprinklers, which are not obligatory at privately-run Quebec facilities where residents are mobile.

Of the 52 residents, 27 were age 85 or older, and some needed walkers or wheelchairs to get around.

Bernatchez cautioned against pointing fingers at anyone, saying now was the time for relatives to mourn together.

"I don't think people are going to stop and go looking for whoever is to blame," she said.

"They've already been through enough. It's not helpful to agitate them more. Let's let the investigators do their work and come up with solutions," she said. (Editing by Jeffrey Hodgson and Jan Paschal)